For a number of years, powder coating materials have been much used for coating surfaces. These coating materials are finely divided polymeric powders which can be applied by a variety of techniques. For example, the powder coating material can be sprayed by electrostatic charging onto the part that is to be coated, and which carries an opposite charge, in which case a thin film is formed which is cured by heating to the powder's softening point. Another possible means of application is that of tribostatic application (charging by friction in the spraygun). Powder coating materials possess the advantage over conventional, solvent-based coating systems of causing no solvent emissions whatsoever and, by virtue of their recyclability, of having virtually no overspray. Powder coating materials can be pigmented with color pigments and effect pigments such as metallic effect pigments or pearlescent (or pearl luster) pigments in order to achieve decorative effects.
Common organic or inorganic pigments and also customary powder coating ingredients, such as binders, curing agents, fillers, additives, etc., are first premixed in a mixer and then extruded. The chips obtained in this procedure are ground in a pinned-disk mill or impact-lining mill or similar equipment.
Where platelet-shaped effect pigments, such as pearlescent pigments or metallic effect pigments, for example, are used in powder coating materials, they must be incorporated into the powder coating material in a relatively gentle process. Otherwise the pigments, which are sensitive to shearing, undergo fracture and so lose their characteristic properties.
Two processes of incorporation are conventional in industrial practice. In the case of the first process, the dry-blend process, the powder coating particles and the effect pigments are simply mixed with one another. A disadvantage of this process is the presence of powder coating particles and effect pigment particles as separate entities. Owing to the difference in shape and chemical nature of powder coating particles and effect pigments, separation phenomena are frequently encountered in the course of electrostatic application. Accompanying this is a loss of recyclability of the unapplied powder coating material, owing to a relative change in the proportions of powder coating particles to effect pigments in the powder coating material. Re-using the unapplied powder coating material in a new powder coating would lead to a coating film of altered quality. Furthermore, in the powder coating gun, particle aggregates may develop, which lead to large aggregates (“spits”) on the coating and hence to an unacceptable visual appearance. The second process is referred to as bonding. Here, in the presence of the pearlescent pigment, the powder coating material is heated to just a little way below its glass temperature and hence is partially melted. This results in physical adhesion of the pearlescent pigments to the powder coating particles and hence in the formation of composite particles. By this means, as a general rule, the disadvantages accompanying dry blending are alleviated. However, the bonding operation is generally incomplete: within the powder coating there remain individual pearlescent pigments not joined to the binder. A further disadvantage is that this process is applicable only poorly in the case of fractions with relatively large pearlescent pigments. Here, in comparison to the powder coating binder particles, the pearlescent pigments are so large that only some adhere firmly enough.
EP 0 721 005 B1 describes pigment preparations comprising pearlescent pigments, a phosphate derivative, and spherical particles. Such preparations are said to bring advantages particularly in the printing sector. Applications in powder coating materials or bronze-effect finishes have not been described.
Pearlescent pigments coated with long-chain saturated fatty acids are disclosed in EP 0 285 977 B1. Such pearlescent pigments possess leafing properties and can be used with advantage in bronze-effect finishes.
Pearlescent pigments coated with alkylsilanes and having leafing properties are described in EP 0 492 223 B1. These pigments are to be used for inhibiting the yellowing of pigmented plastics.
These two known surface-modified pearlescent pigments having leafing properties, however, exhibit disadvantages in powder coating materials. The optical effects such as luminance or flop are not substantially enhanced. Only at very high levels of fatty acids or alkylsilanes is a strong leafing behavior and hence a greater visual brilliance obtained. Here, however, the abrasion resistance is deficient. Alkylsilane-coated pearlescent pigments cannot be applied, furthermore, with sufficient reproducibility.
DE 197 08 167 A1 discloses a pigment preparation comprising, among other components, surface-active substances, such as fatty acids and fluorosurfactants. The use of surface-active substances is intended to produce nondusting, homogeneous pigment preparations.